AFS Library

MICROSEGREGATION IN CASTINGS AND INGOTS

3537
Journal: Transactions of the American Foundrymen's Society V 72 P 353-362, 1964 (10 p)
Author: Flemings, M C

Recent research on microsegregation in castings and ingots at Massachusetts Institute of Technology is summarized. It is shown that simple mathematics are adequate to approximately predict the severity of segregation, as measured by segregation ratio (ratio of maximum to minimum composition over distances the order of dendrite arm spacings). The major source of error in the simple analysis is that it neglects diffusion in the solid during freezing. More detailed analyses which take this diffusion into consideration agree quantitatively with experiment. Also in agreement with experiment, segregation ratio is shown to be nearly independent of rate of solidification over a wide range of rates of solidification, even when diffusion in the solid is an important factor. Dendrite morphology is briefly discussed because of its influence on the form of microsegregates; at least in the latter stages of solidification, dendrites prefer a platelike morphology. Homogenization of castings is considered in the light of the foregoing and it is shown to be extremely difficult, in all but rapidly solidified castings to achieve complete homogenization. Sample calculations are given for low alloy steel. It is shown, for example, that a low alloy steel casting solidified at moderate rate requires at least one hr at temperatures in excess of 2500 F (or much longer times at slightly lower temperatures) to achieve significant homogenization of elements other than carbon. Considerable difficulty is also experienced in obtaining fully homogeneous wrought material, because of dendritic segregation in the original ingot. Mechanical properties of both ferrous and nonferrous castingg are substantially improved by intensive homogenization treatments.



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